Neuroprosthetics is an area of neuroscience concerned with neural prostheses, developing artificial devices to replace or improve the function of an impaired nervous system. The neuroprosthetic seeing the most widespread use is the cochlear implant, with approximately 100,000[1] in use worldwide as of 2006.

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An early difficulty in the development of neuroprosthetics was reliably locating the electrodes in the brain, originally done by inserting the electrodes with needles and breaking off the needles at the desired depth. Recent systems utilize more advanced probes, such as those used in deep brain stimulation to alleviate the symptoms of Parkinsons Disease. The problem with either approach is that the brain floats free in the skull while the probe does not, and relatively minor impacts, such as a low speed car accident, are potentially damaging. Some researchers, such as Kensall Wise at the University of Michigan, have proposed tethering 'electrodes to be mounted on the exterior surface of the brain' to the inner surface of the skull. However, even if successful, tethering would not resolve the problem in devices meant to be inserted deep into the brain, such as in the case of deep brain stimulation [DBS].

Research has also been undertaken by the American CIA in the 1950s as part of the MKULTRA program, although it is uncertain whether this meets the definition of neuroprosthesis. Examples: Subproject 86 (developing an invasive prosthetic identifier thought to expand in reporting body responses such as blood pressure or tremor), subproject 94 (neurostimulus in animals immediately defining direction of movement and control) and subproject 119 (remote "reassembly" or unification of monitored neural impulse into a useable data product).

One of the prominent goals in neuroprosthetics is a visual supplement, noting roughly 95% of all people considered 'blind' suffer significant impairment but have some capability (for example, seeing some sort of blur) - only about 5% of 'blind' people are totally blind. By the 1940s, researchers had established the concept of artificial electrical stimulation of the visual cortex, and in the late 1960s, British scientist Giles Brindley produced breakthrough findings with a system for placing electrodes on the brain's surface. When specific areas of the brain were stimulated in blind volunteers, all reported "seeing" phosphenes that corresponded to where they would have appeared in space. However, experiments were discontinued because of the uncomfortably high currents required for stimulation on the surface of the brain.

Encouraged by this work, the National Institutes of Health undertook a project to develop and deploy an interface based on ultrafine wire (25 to 50 micrometres) densely populated with electrode sites that could be implanted deep into the visual cortex, thus requiring less current than Brindley's original design. This work led to new electrode technology—finer than the width of human hair—that could be safely implanted in animals to electrically stimulate, and passively record, electrical activity in the brain. The efforts produced significant advances in neurophysiology, with publication of hundreds of papers in which researchers attempted to develop an electronic interface to the brain.

With this new technology, several scientists, including Karin Moxon at Drexel, John Chapin at SUNY, and Miguel Nicolelis at Duke University, started research on the design of a sophisticated visual prosthesis. Other scientists have disagreed with the focus of their research, arguing that the basic research and design of the densely populated microscopic wire was not sophisticated enough to proceed.

In 2003 Philip Kennedy (Emory and Georgia Tech) had an operable if somewhat primitive system which allowed an individual with paralysis to spell words by modulating their brain activity. Kennedy's device uses two neurotrophic electrodes: the first is implanted in an intact motor cortical region (e.g. finger representation area) and used to move a cursor among a group of letters. The second is implanted in a different motor region and used to indicate the selection. [2]

More recently, there have been successful experiments completely replacing lost arms with cybernetic replacements by using nerves normally connected to the pectoralis muscles. These arms allow a slightly limited range of motion, and reportedly are slated to feature sensors for detecting pressure and temperature.[3]

Medtronic and Advanced Bionics are significant commercial names in the emergent market of Deep Brain Stimulation. CyberKinetics is the first venture capital funded neural prosthetic company, and has the first human trial.